This invention relates to personal care and to certain alkylmethyl polysiloxanes useful in skin care applications. More particularly the invention is directed to moisturization and to the formation of films on the skin which function as barriers in order to reduce transepidermal water loss with the result that skin is softened by virtue of its own moisture.
In a recent publication by Th. Goldschmidt AG dated July 1989 and entitled "ABIL.RTM. Silicones" it is reported that certain polysiloxane polyalkylene copolymers known as ABIL.RTM.-WAX 9800 and ABIL.RTM.-WAX 9801 have utility in skin care applications such as day creams, all purpose creams and body lotions. The materials are said to be soluble in cosmetic oils and waxes and to protect against aqueous media when employed in amounts of one to five percent by weight. These materials otherwise known under The Cosmetics, Toiletries and Fragrances Association adopted names of stearyl dimethicone and cetyl dimethicone have the structural formula: ##STR1##
It has been determined by elemental analysis, functional analysis and gel permeation chromatography that the value of the integer x in the above formula is two and that the value of the integer y is four, that is, the ABIL.RTM. siloxanes are low molecular weight. In contrast to these low molecular weight polysiloxanes, the siloxanes of the present invention have a molecular weight upwards of at least ten times the molecular weight of the ABIL.RTM. silicones. As a result the siloxanes of the present invention are less volatile, mope substantive to the skin surface, and form a film on the skin which functions as a barrier to prevent the permeation of moisture upward from the skin and through the film. This reduces the transepidermal water loss from the skin with the result that the skin is softened due to the retention of mope of its own moisture. Test data indicate that the siloxanes of the present invention possess these improved properties to an unexpectedly greater degree than the ABIL.RTM. silicones as will be shown hereinafter.
U.S. Pat. No. 4,574,082 issued Mar. 4, 1986 describes cosmetics containing a dimethylpolysiloxane in admixture with an organopolysiloxane such as polymethyloctylsiloxane and polymethyloctadecylsiloxane. Where these materials are described however they are indicated to be polymers rather than copolymers. In addition, according to the present invention it has been discovered that treatment with a composition that is free of dimethylpolysiloxanes retains mope moisture. This is for the reason that a film of dimethylpolysiloxane allows the permeation of water vapor through the film and therefore dimethylpolysiloxanes suffer from the disadvantage of being ineffective moisture barriers. By eliminating dimethylpolysiloxanes and applying films containing only silicones which are high molecular weight alkylmethylsiloxanes, it has been possible in accordance with the present invention to reduce substantially transepidermal water loss from skin-like collager material due to the presence of dimethylpolysiloxanes and to therefore provide occlusive moisturizing treatments. Such occlusive moisturizing treatments ape not possible with the cosmetic products of the '082 patent.
Protective skin creams are described in United Kingdom Patent No. 737,134 granted Sep. 21, 1955. These skin creams are said to include certain hydrocarbon substituted organosiloxanes. The hydrocarbon radicals are noted preferably to be lower alkyl radicals such as methyl, ethyl and propyl however the British patent indicates that for certain uses higher alkyl radicals such as octadecyl and lauryl radicals may be desired. This description allows the presence of higher alkyl radicals at the ends of the polymer chain rather than along its backbone as in the present invention, and fails to describe copolymeric materials.
Skin care lotions and creams said to include mixed C.sub.1 -C.sub.3 alkyl polysiloxanes such as methylethylpolysiloxane are disclosed in U.S. Pat. No. 4,960,764 issued Oct. 2, 1990. Siloxanes including short chain alkyl groups such as ethyl and propyl do not differ significantly from dimethylpolysiloxanes and therefore the siloxanes of the '764 patent suffer from the disadvantage of being ineffective moisture barriers as explained above in detail. Thus methylethylpolysiloxane and methylpropylpolysiloxane will each possess a high water permeability through their respective film and will each be incapable of functioning as effective occlusive barriers in comparison to the high molecular weight C.sub.6 to C.sub.30 alkylmethylsiloxanes of the present invention. Further short chain alkylmethylpolysiloxanes as described in the '764 patent are less durable and their films may be easily removed from the surface of the skin in contrast to the substantive nature of the films formed by the materials of the present invention.
Accordingly new and novel personal care formulations are provided herein in which an alkylmethyl polysiloxane is utilized as a substantive barrier to water loss to provide soft skin.